i am not clear on your request sorry are you asking how you can temporarily disable the back key on the browser? or disable the backspace key as a browser back key?

Hi
At 1st,i have to say thank you for attention, i am sorry, I think its not a clear question ,i disable the backspace in a .aspx page, but i need to active it again by using some codes in a button body, then the user can click on the button, instead of using backspace key that may use backspace accidentally, in other words he wants and is sure needs to be back in pages and its not a simpatico usual using bkspace, then the users' error to submit a filled form in this way may occurred less than often, in fact my goal is avoid submit a filled form more than one time by inactive bkspace except some times its needed that would bkspace allowed thru a coded Button not bkspace icon/key.

i think i am clear now you disable the key normally but when you have a field they need to fill in you want to enable it again? I would add two events to each textbox client side. The first is on enter call your javascript function you have but this time enable the key. The second would be on leave to call the javascript function you have that disables the key. That way you know it is only active for that textbox and you wont have to worry about other problems. You should be able to use the script you have already so i wont post one here.
The downside is you have to do it for each textbox, but it does give you more control (for example you wouldnt want to enable it on the form or it may go back when they are not in a textbox and hit the backspace key).

Hope it helps. Let me know if you need anything more